bodysuit
A skin tight, one-piece garment that covers the body from the neck to at least the upper legs.
Note that it has to be one piece of clothing, not separate pieces of clothing, such as a leotard over a pair of pantyhose.
If it covers only the upper body and not the legs, like a one-piece swimsuit, use leotard instead. If the fabric is obviously sheer or thin, use bodystocking instead.